1. Field of the Invention
This invention relates to methods and apparatuses for adjusting the maximum, or saturation, laser or LED power in a printing apparatus. More specifically, this invention is directed to methods for controlling the exposure level based on the type of image object currently being exposed.
2. Related Art
Prior to the advent of high quality computer generated page images, page images such as those found in newspapers, newsletters, magazines and the like were formed by graphic artists composing the page images by hand. Thus, each different type of image object on a page image, including text, photographs, constant color areas or graphs such as pie charts, and sampled or continuously changing images such as sweeps, was optimally formed, independently of any other type of image object.
Because these page images, comprising one or more of these types of image objects, were composed by hand, each type of image object was inherently treated independently of the other image objects. Thus, the optimal halftone screen design for photographs, which differs from the optimal halftone screen designs for constant color areas and text could be optimally selected and the screen arranged to an optimal angle. Likewise, such optimal treatment of each type of object could be obtained.
With the advent of digital color workstations, copiers and printers, creators of page images who had previously relied on graphic artists to compose and print page images could instead create, compose and print the page images on their own using a computer connected to the digital color copier/printer.
However, such prior art digital systems for creating a page image, for decomposing the page image into print engine instructions and for controlling the print engine treated a page image as a single, unitary image. Thus, in a page image optimized for text and/or lineart, when a high frequency halftone screen is used, the text portion of the page image will be quite sharp. However, a constant color portion of the page image will contain an obvious mottling from printer noise. In addition, the sampled color portion and the sweep portion of the page image will have obvious contouring due to the lack of sufficient gray levels available with the high frequency screen.
In a page image optimized for a large constant color portion, a halftone screen specifically designed to hide printer instabilities will produce a high-quality, text-free and artifact-free constant color area. However, sharpness of text will decrease and the gray values for each tint will not be well-related, so sampled color portions and sweep portions would be unacceptable. The gray levels will not step smoothly from one to the next because each dot level is designed separately without relation to the other levels.
In a page image optimized for sampled color and sweep portions, the sweep portion and the sampled color portion will have a higher quality because a low frequency halftone screen is used with more gray levels available. However, text will be low in quality and constant color portions will have an obvious texturing.